In a conventional gas turbine engine, a compressor introduces air into a combustion chamber in which the air is mixed with the burning fuel to produce gases that drive a turbine. The turbine drives a load consisting of the compressor and an external load. The efficiency of such a turbine design improves with increasing operating temperatures; however, there is a limit to the operating temperature dictated by the temperature at which the turbine blades and related systems fail.
To maintain the temperature below this maximum temperature, the fuel to air ratio in the combustion chamber is maintained below the point at which stoichiometric combustion of the fuel is achieved. The additional air maintains the gases below the maximum operating temperature. Unfortunately, the energy needed to compress this additional air reduces the overall efficiency of the engine.
These observations have led to gas turbine designs in which steam and/or water is injected into the combustion system. For example, Dah Yu Cheng (U.S. Pat. Nos. 3,978,661, 4,128,994 and 4,297,841) recognized that steam added to the Brayton cycle can significantly increase the power and efficiency of the engine provided heat is recovered from the exhaust gases. The power generated by the drive turbine at any given temperature is determined by the specific heat of the gases expanding through the turbine. Since steam has a much higher specific heat than air, the use of steam as the coolant significantly improves the power that can be generated by the turbine.
Unfortunately, the amount of heat that leaves the system in the exhaust gases also increases when steam is used. The exhaust gases generated in a steam injected engine leave at a higher temperature and have a higher specific heat. Hence, in the absence of some form of heat recovery system, the overall efficiency of the engine decreases. Cheng used a heat recovery boiler to recover the heat from the exhaust gases of the turbine to produce steam. Because of the pinch point limitation on the operating pressure of the heat recovery boiler, and hence the operating pressure ratio of the turbine, the maximum achievable efficiency was not achieved in this system. Patton and Shouman (U.S. Pat. No. 4,841,721) solved the pinch point problem by operating the heat recovery system in the supercritical regime. Urbach et al. in U.S. Pat. No 4,509,324 described an engine system having a different heat recovery system. In this patent the exhaust gases from the compressor turbine are mixed with the high pressure gases from the combustor before entering the power turbine. As a result, there is a significant reduction in the available power.
One of the major obstacles in implementing the Cheng, Patton-Shouman, and Urbach et al. systems is the cost of developing new engine components and new engines. When the scheme described by Cheng is applied to an existing gas turbine plant, the operating range of the plant is limited by the surge characteristics of the compressor. As a result, the amount of steam that could be injected is limited to about 10% to 15% of the air mass flow into the engine combustor. This level is significantly below the optimum steam injection level. To overcome this limitation, one would need to replace the drive turbine with a turbine capable of expanding a much larger mass flow. The cost of such a redesign and retro-fit is prohibitive. Hence, the improvements obtainable by steam injection have not been realized in existing power plants.
Broadly, it is the object of the present invention to provide an improved gas turbine engine.
It is a further object of the present invention to provide a retro-fit to existing gas turbine engines that allows the benefits of steam injection to be obtained without requiring that new turbines be designed and constructed.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.